1. Field of the Invention
This invention relates to a magneto-optical recording medium. This invention particularly relates to a magneto-optical recording medium which reflects light such that little difference in phase of the reflected light occurs, whereby elliptic polarization of the reflected light is minimized and fluctuation in the C/N ratio among apparatuses for driving the magneto-optical recording medium is reduced.
2. Description of the Prior Art
In recent years, magneto-optical recording media have been used widely in the form of magneto-optical disks, which are used for large-capacity information storage or the like and which enable the writing and reading of information with a laser beam.
In general, a magneto-optical recording medium comprises a transparent substrate, which is constituted of a resin such as polycarbonate, glass, or the like, and a magneto-optical recording film overlaid on the substrate. The magneto-optical recording film comprises protective dielectric layers and a recording layer which are formed with a vacuum film-forming process, for example, a sputtering process.
The recording layer is constituted of an amorphous alloy which mainly contains a rare earth metal, such as Tb, Nd, Dy, or Gd, and a transition metal, such as Fe, or Co.
Ordinarily, protective dielectric layers are provided in order to protect the recording layer and enhance its characteristics.
In order for the magneto-optical recording film to exhibit high sensitivity and a high C/N ratio, the magneto-optical recording film is composed of three layers. Specifically, in cases where light is irradiated to a magneto-optical recording medium from the side of the substrate during the reading of information from the magneto-optical recording medium, a first protective dielectric layer, a recording layer, and a second protective dielectric layer are overlaid in this order on the substrate. Such a configuration enhances the Kerr rotation angle through multiple beam interference occurring at the first protective dielectric layer.
Also, as disclosed in, for example, Japanese Unexamined Patent Publication Nos. 55(1980)-87332 and 57(1982)-120253, it has been proposed to employ a reflection layer type of magneto-optical recording medium in order to obtain a good recording bit pattern and keep the C/N ratio high. By way of example, the reflection layer type of magneto-optical recording medium comprises a magneto-optical recording film composed of three layers as described above, and a reflection metal layer overlaid as a top layer on the side opposite to the substrate.
In general, when light impinges upon a magneto-optical recording medium and is reflected therefrom, the Kerr effect occurs. Specifically, the plane of polarization of the light rotates by a certain angle (a Kerr rotation angle), and the reflected light becomes polarized elliptically. This is because, when light is reflected from the magneto-optical recording medium, a phase difference .PHI. occurs with the reflected light.
If a phase difference occurs with the reflected light, the carrier output and the C/N ratio will decrease.
As described above, a phase difference occurs when light is reflected from a magneto-optical recording medium. A difference in phase of reflected light is also caused to occur by an optical element of a pickup device of an apparatus which drives a magneto-optical recording medium. Specifically, a pickup device is provided with a reflection optical element, such as a 45-degree mirror or a beam splitter, which guides light reflected from a magneto-optical recording medium. The reflection surface of the reflection optical element is ordinarily constituted of a plurality of dielectric layers, which cause a difference in phase to occur with light reflected therefrom.
A phase difference .delta. of the reflected light, which phase difference is caused to occur by an optical element of a pickup device, varies largely among pickup devices. Therefore, even if the same magneto-optical recording medium is used, the C/N ratio obtained will vary, depending on the apparatus which drives the magneto-optical recording medium.
The variance in the C/N ratio among the drive apparatuses increases as the phase difference .PHI. of the reflected light, which phase difference is caused to occur by a magneto-optical recording medium, is larger.
Specifically, in general, the phase difference .delta. of the reflected light, which phase difference is caused to occur by an optical element of a pickup device, takes one of various values around zero degree. When the phase difference .PHI. of the reflected light, which phase difference is caused to occur by a magneto-optical recording medium, is closer to zero degree, the variance in the C/N ratio among pickup devices becomes smaller.
Therefore, in order for the problem with regard to the variance in the C/N ratio among the drive apparatuses to be eliminated, it is important that the phase difference .PHI. of the reflected light, which phase difference is caused to occur by a magneto-optical recording medium, be minimized.
However, any means efficient for minimizing the phase difference .PHI. of the reflected light has not heretofore been proposed.